ABSTRACT Depression is debilitating, costly, and highly prevalent, representing the leading cause of ill health and disability worldwide. A majority of US households includes pets. Pets offer a source of emotional support and previous research has shown that they may help ease depression, anxiety, and stress. However, longitudinal studies to establish causal relationships or explore mechanistic pathways are still lacking. The objective of this proposal addressing PAR-18-213, ?Human-Animal Interaction (HAI) Research?, is to, for the first time longitudinally, examine the effects of pets in the home on depression and psychological distress in humans, and to explore potential pathways by studying stress biomarkers (salivary cortisol-DHEA ratio) and the direct and mediating effects of the gut microbiome and metabolome in these associations. We will leverage three ongoing large population-based studies ? the Nurses? Health Study 2 (NHS2, n=116,430), NHS3 (n=47,435), and the Growing Up Today Study (GUTS), 12,413 children of the NHS2 women, who were 9-14 years old at enrollment in 1996 (88% with pets). With biennial follow-up, they offer a wealth of repeatedly assessed lifestyle, environmental, and psychosocial assessments allowing to finely control for their potential confounding in our analyses. Our primary study aims include an ongoing collection of stool samples from 35,000 participants in the NHS2, who we will follow after stool collection for depression to include 400 women with/without pets and incident depression in the current application. In addition, a subcohort of 226 NHS2 participants (62% with pet) collected detailed, repeated psychosocial assessments including information on pet attachment, pet owner?s personality dimensions, and stress biomarkers as well as participants? gut microbiome; thus, we leverage these microbiome data at no additional cost. Further, embedded in NHS3, we will initiate a longitudinal ?Nurses? Pets Study? (NPS), enrolling pets of nurses into active follow-up for future HAI studies. Our chief hypotheses are that pet ownership/attachment reduces risk of incident depression and psychological distress, and that primary pathways include an improved cortisol-DHEA ratio and alterations in the gut microbiome, offering the potential for new therapeutic approaches and evidence- based advice regarding the introduction of a pet into one?s home.